Direct gasoline injection has some distinct advantages over prior art systems with respect to emissions and fuel economy of the engine, mainly because of an increase in the efficiency of the engine.
Currently, efforts are underway to develop a reliable and inexpensive pump capable of generating a relatively high pressure (such as 120 bar and higher) required for supplying a common rail system used in direct gasoline injection. One such pump is shown in U.S. patent application Ser. No. 09/031,859, filed Feb. 27, 1998 and entitled "Supply Pump for Gasoline Common Rail" (International Application No. PCT/US99/03830 published under International Publication No. WO 99/43949), which is assigned to the present assignee hereof and the entire contents of which is hereby incorporated herein by reference. This supply pump, as is typical of pumps in general, includes a rotating shaft having bearings that are lubricated by either a lubrication fluid (oil) or a combustion fluid (fuel) disposed on the low pressure side of the pump. The fuel may be pre-pressurized to 3 or 4 bar by a separate feed pump, e.g., remotely located in a fuel tank. Seals, such as lip seals, which extend radially about the rotating shaft, are employed to prevent escape and/or mixing of either fluid.
While the supply pump described in the International Publication is suitable for its intended purposes, a problem can occur with the supply pump in that because of the differences in pressure between the oil pressure and fuel pressure within the pump, the lip seals may be canted one way or the other into contact with the rotating shaft resulting in premature wear thereof.
Another problem can also arise because of the difference in pressure between the oil and the fuel. In particular, passage by either the oil or the fuel through the seal occurs, due to the canting of the seal and/or other factors, resulting in improper mixing of these fluids. In one direction, mixing of the fuel into the oil may result in a reduction in lubricity of the oil. It will be appreciated that reduced lubricity of the oil can, for example, result in premature wear of the pump and possibly other systems of the engine. Also, potential hazardous waste problems concerning disposal of the oil/fuel mixture may arise. In the opposite direction, the mixing of the oil with the fuel may result in a reduction in engine performance.